The present invention relates to a semiconductor device and, more particularly, to a semiconductor device having a gate insulating film.
The gate insulating film for a transistor formed over a semiconductor integrated circuit device is formed between a gate electrode and an active area under the gate electrode. The gate insulating film is required to inhibit the generation of a gate leakage current caused by a tunneling current from the gate, while also being required to be capable of performance to generate or not to generate a channel in the active area according to a change in an electric field caused as the gate electrode is switched between an ON state and an OFF state.
The performance of a transistor depends on the electrostatic capacity of the gate insulating film used therein. Where the capacitance of the parallel plate capacitor formed between the gate electrode and the channel is represented by C, the dielectric constant of the insulating film is represented by k, the area as seen from above of the gate electrode is represented by A, and the thickness of the insulating film is represented by d, a relationship of C=kA/d is established. To reduce the chip area and promote transistor miniaturization, the electrode area A as seen from above needs to be reduced. To reduce the electrode area A of a transistor without sacrificing its performance requires the insulating film thickness d of the transistor to be reduced compared with the electrode area A.
Paraelectric materials, for example, silicon oxide films have been in use as gate insulating films. As semiconductor integrated circuits are increasingly miniaturized and high integrated, however, gate insulating films have come to be as thin as a total thickness of several atoms, causing a problem of leakage current leaking through the insulating film due to a quantum tunneling effect. To be more concrete, in a case where a field effect transistor of the so-called 28-nm generation having a gate electrode length (gate length) of about 28 nm has a silicon oxide film used as a gate insulating film, the film thickness is about 2 nm. The film thickness is equivalent to a total thickness of several atoms, that is, it is extremely thin. This results in increasing the leakage current leaking through the gate insulating film due to a quantum tunneling effect. Also, when gate insulating films are made extremely thin, variations in thickness of the gate insulating films of plural transistors formed on a same substrate enlarge making transistor formation difficult.
Conversely, increasing the thickness d of gate insulating films decreases variations in thickness among plural gate insulating films. To realize a transistor having a gate insulating film with an increased thickness d and performance equivalent to that of a comparable transistor having a gate insulating film with an unincreased thickness d, it is necessary to increase the electrode area A of the transistor with the increased thickness d of the gate insulating film. To be more concrete, when, for a transistor of the 28-nm generation having a gate insulating film with a thickness d of 2 nm, the thickness d is increased to 50 nm, the voltage applied to the active area of the transistor is reduced to 1/25. With the voltage largely reduced, switching the gate electrode to an ON state cannot cause any channel to be formed in the active area, that is, the transistor is disabled. To have a channel formed by applying a voltage, as done for the transistor having a gate insulating film with a thickness d of 2 nm, to the transistor having a gate insulating film with a thickness d increased to 50 nm, it is necessary to increase the gate area A of the transistor 25 times. This means that the gate electrode whose planar shape is rectangular needs to be enlarged five times both along a planar direction (x-direction) and along another planar direction (y-direction) approximately orthogonal to the x-direction. This results in a transistor gate length of about 140 nm and makes the transistor equivalent to a five-generation older transistor.
Increasing the thickness d of the gate insulating film while maintaining the electrode area A increases the aspect ratio determined by the thickness of the whole gate including the gate insulating film and the gate electrode and the gate length. To be more concrete, in the case of a transistor of the 28-nm generation, for example, the gate insulating film thickness is about 2 nm, the gate thickness, including the thicknesses of the gate insulating film and a gate electrode, is about 50 nm, and the distance between a pair of adjacent gates is about 65 nm based on a planar view. In this case, the aspect ratio determined by the total gate thickness and the gate length is about 2. If the gate insulating film thickness is increased to 50 nm while leaving the other dimensions unchanged, the aspect ratio increases to about 4. Generally, the mechanical durability of a gate electrode is secured by forming side wall insulating films to be in contact with the side walls on both sides of the gate electrode. Increasing the aspect ratio to about 4 may, however, cause the gate electrode to be broken by the stress generated when the gate electrode is chemically mechanically polished during gate processing.
Recently, instances have been reported in which a gate insulating film made of a high dielectric constant material called “high-k” is used to address the above problem. A high dielectric constant material is paraelectrics having a large dielectric constant k, and using a gate insulating film made of a high dielectric constant material can increase the capacitance C of a parallel plate capacitor included in a transistor, making it possible to widen the distance between the gate electrode and the active area. Namely, the gate insulating film can be made thicker. A thick gate insulating film allows, when the gate electrode is in an ON state, an adequately strong electric field to be applied to the active area and, when the gate electrode is in an OFF state, a physically large distance to be secured between the gate electrode and the active area so as to reduce leakage current attributable to a tunneling effect. As a result, the transistor, in which gate leakage current is reduced compared with prior-art transistors, can be operated consuming less power than before.
However, when a gate insulating film made of a high-k material is used in combination with a gate electrode made of polycrystal silicon, trouble tends to develop at an interface (contact surface) between the gate insulating film and the gate electrode to cause rising of the operating voltage. In addition, phonon vibration may internally occur to impede the flow of electrons.
The dielectric performance of a gate insulating film can be enhanced also by using a ferroelectric material instead of a high dielectric constant material. For example, as disclosed in Japanese Unexamined Patent Publication No. 2001-332125, the variation in dielectric constant of a ferroelectric material caused by temperature changes can be reduced by using a gate insulating film including two ferroelectric materials of different compositions. Also, as disclosed in Japanese Unexamined Patent Publication No. Hei 11(1999)-204744, by including a small amount of titanium in a ferroelectric film used as a gate insulating film, the increase in leakage current in the gate insulating film can be inhibited and, hence, the variation in dielectric constant of the ferroelectric film caused by temperature changes can be reduced.
On the other hand, in Japanese Unexamined Patent Publication No. 2008-205284, an organic field effect transistor (FET) having a gate insulating film made of an antiferroelectric film is disclosed. Furthermore, capacitors using an antiferroelectric film as an insulating film are disclosed, for example, in Japanese Unexamined Patent Publication No. 2001-222884 and Japanese Unexamined Patent Publication No. 2000-243090.